During the manufacture of insulated wire, it is the usual practice to apply the insulating rubber or polymer layer by extruding it over the wire core and then subjecting the layer to a vulcanizing treatment. In the past, saturated steam has been used as the heating agent for cross-linking rubber-like plastics on the electrical wire. In order to operate properly, it has been common in the trade to utilize a pressure of at least 15 kg/sq.cm. to maintain a temperature of about 200.degree. C. This aspect of the prior art is described in the patent of MIYAUCHI No. 3,513,228; the patent of CHRISMAN No. 3,901,633 describes a similar process, using pressurized gas. Another method of vulcanizing that has been used in the past is that of using a conductive coating, as is shown and described in the Defense Publication of DAY No. T-905,001, and patents of LINDSAY No. 2,458,864, and of HOPPE No. 3,387,079. A method of vulcanizing that uses the heating of the wire core by passing an electrical current through it is described in the patent of KITSEE No. 768,755.
In some prior art systems, the wire core itself is heated in the manner shown in the patents of BALTHIS No. 2,308,638, of FLAHERTY No. 3,263,268, and of STINGER No. 4,117,189. The use of radiant heat for the vulcanization is shown and described in the patents of MIYAUCHI No. 3,513,228 and 3,635,621, as well as the patent to NAKAMURA No. 3,588,954 and the patents of KARPPO No. 4,035,129 and 4,155,695. Ultrasonics have been suggested for this purpose, as described in the patent of SENAPATI No. 4,548,771. Considerable work has been done in the past with respect to vulcanizing by the use of high-frequency magnetic fields and of microwave energy. This work has been described in the patents of DODGE No. 3,354,243 (metal powder in the polymer), of MENGES No. 4,372,898, of BABBIN No. 4,512,942, of SCHULZ No. 4,514,162, of WANG No. 4,626,642, of THORSRUD No. 4,661,299, of SEJIMO NO. 4,702,867, of NAKAGAWA No. 4,497,759, of GREENE No. 3,642,532, and of ITOH No. 4,481,159.
These prior art methods and apparatus have suffered from a number of disadvantages. The steam system, for instance, supplies the vulcanization heat through a steam pipe that is 250-500 feet in length. The steam must be generated and the pressure regulated, resulting in a high insulation and maintenance cost for the equipment; vapor tends to become trapped in the polymer layer, thus impairing the insulation dielectric strength. The production speed is dictated by the steam pipe length and it is difficult to run small-sized wire, because of the likelihood of breakage in such a long piece of equipment. Furthermore, when pressure is used, the polymer tends to be pressed into interstices, particularly in stranded wire, so that the finished insulated wire is difficult to strip during installation in the field. The other methods have similar drawbacks. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a method for vulcanizing wire insulation in which heat is transferred by radiation, so that it is not necessary to heat a medium, as in the case of steam or gas.
Another object of this invention is the provision of a vulcanization system in which radiation penetrates the material, thereby giving a uniform heat distribution inward from the surface.
A further object of the present invention is the provision of wire insulation vulcanization in which the heat is readily available and there is no waiting time and no medium that must be heated and pressure-stabilized.
A still further object of the invention is the provision of a insulated wire vulcanization system, in which the path is relatively short, so that no problems are encountered in the production of fine wire.
It is a further object of the invention to provide apparatus for the vulcanization of insulated wire, which apparatus is simple and rugged in construction, which can be manufactured inexpensively, which is capable of a long life of useful service with a minimum of maintenance, and which provides easy and responsive control of temperature.
Another object of the invention is the provision of a vulcanization system that does not use a vapor medium, so that moisture or gas bubbles are not formed and the dielectric strength of the insulation is uniform; no pressure is involved, so that the insulation is not pressed into interstices and the finished product is easily stripped in the field.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.